1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for making a high speed scanning such as an echo planar imaging sequence, and more particularly to a multi-slice divisional-scanning type magnetic resonance imaging apparatus which acquires magnetic resonance echo signals (hereinafter referred to as echo signals) for a single slice in several scans in order to improve spatial resolution and acquires echo signals from multiple slices in a single pulse sequence in order to obtain three-dimensional information of an object under examination.
2. Description of the Related Art
In a prior art of such a magnetic resonance imaging apparatus (hereinafter referred to as MRI apparatus), as shown in FIG. 1, a single slice is divided into two or more regions (four regions "a", "b", "c", and "d" in this example) on the Fourier space or the k space, and an echo signal is acquired from each of the regions. It is assumed that an imaging volume of an object under examination is comprised of eight slices #1 to #8. Then, the regions "a", "b", "c" and "d" of the slice #1 on the k space are first scanned in sequence, as shown in FIG. 2. Next, the slice #2 is scanned in the order of the regions "a", "b", "c", and "d". Likewise, data for the regions "a", "b", "c", and "d" are acquired in sequence for each of the slices #3 to #7. Finally, data for the regions "a", "b", "c", and "d" of the slice #8 are acquired. The echo signal thus acquired for each of the slices is then subjected to two-dimensional Fourier transformation, thereby allowing a magnetic resonance image of each slice to be reconstructed. Having an advantage that echo signals can be acquired from multiple slices by the use of a single pulse sequence, the prior art suffers from a drawback that the scan time is long.
In general, MRI apparatuses suffer from a major problem that the scan time is long. From a consideration of deterioration in image quality due to physiological motion of a patient being examined, such as motion due to respiration, it is preferred that the scan time should be as short as possible. In the case of the multi-slice divisional-scanning method described above, however, in order to shorten the scan time, it is required to shorten the RF-pulse repetition time TR. If the repetition time TR were made short, transverse relaxation enhancement images (T2 contrast images) could not be obtained because of the influence of longitudinal relaxation, and the signal-to-noise ratio would also be degraded.